A typical telephone key system comprises a central control unit--i.e. a private branch exchange--, and a multiplicity of remote key stations. It is usual to supply power to each key station along communication lines between the branch exchange and each key station. In the conventional system it is usual to derive power from A.C. mains supply by means of a rectifier/regulator incorporated in the branch exchange. High Tension regulated DC voltage--usually 75-50 volts dc is then fed to each station and converted to logic level Low Tension dc voltage by means of a series regulator incorporated in each key station.
In such a system as aforesaid it is a disadvantage that key stations near to the branch exchange see full voltage and dissipate therefore relatively high levels of power. Those key stations distant from the branch exchange see attenuated voltage, and for these substantially less useful power is available. Also, at distance, the speech/data signals are attenuated. This therefore sets a practical limitation on the system and according to particular design restricts the number of extensions, the number of lamps/facilities, and/or the lengths of line between users.
An alternative approach is to equip each station for direct connection to AC mains supply. This, however, requires bulky and expensive components for each station and increases safety hazards, also requiring a costly solution.